In the field of image display apparatuses, recent years have seen the development and commercialization of flat panel display apparatuses having pixels (pixel circuits) including light-emitting elements arranged in a matrix form. Among such display apparatuses are organic EL display apparatuses using organic EL (electro luminescence) elements as the light-emitting elements of the pixels. The organic EL element is an example of so-called current-driven light-emitting elements whose emission brightness changes with change in current flowing through the element. The organic EL element relies on the phenomenon that the organic thin film thereof emits light when an electric field is applied thereto.
Such organic EL display apparatuses offer low power consumption thanks to their organic EL elements which can be driven by an applied voltage of 10 V or less. Further, organic EL elements are self-luminous. This makes organic EL display apparatuses more advantageous than liquid crystal display apparatuses designed to display an image by controlling the light intensity from the light source (backlight) for each pixel including a liquid crystal cell using the cell. Such advantages include high image visibility and ease of reduction in weight and thickness thanks to no need for illuminating members such as backlight which are necessary for liquid crystal display apparatuses. Further, organic EL elements offer extremely high response speed or approximately several μ seconds. As a result, organic EL display apparatuses produce no afterimage during display of a moving image.
As with liquid crystal display apparatuses, organic EL display apparatuses can be driven by passive or active matrix. It should be noted, however, that although passive matrix display apparatuses are simple in structure, they have disadvantages including difficulties in implementing a large-size, high-definition display apparatus. Therefore, the development of active matrix display apparatuses has been brisk in recent years. In such display apparatuses, the current flowing through the light-emitting element is controlled by an active element provided together with the light-emitting element in the same pixel circuit such as insulated gate electric field effect transistor (generally TFT (Thin Film Transistor)).
Incidentally, the I-V characteristic (current vs voltage characteristic) of organic EL elements is generally known to deteriorate with time (so-called secular deterioration). In a pixel circuit using an N-channel TFT as a transistor adapted to drive an organic EL element (hereinafter described as “drive transistor”) by a current, the organic EL element is connected to the source of the drive transistor. Therefore, secular deterioration of the I-V characteristic of the organic EL element leads to a change in a gate-to-source voltage Vgs of the drive transistor, thus changing the emission brightness of the organic EL element.
A more detailed description thereof will be given below. The source potential of a drive transistor is determined by the operating point of the drive transistor and the organic EL element. In the event of a deterioration of the I-V characteristic of the organic EL element, the operating point of the drive transistor and the organic EL element changes. This leads to a change in the source potential of the drive transistor even if the same potential is applied to the gate of the drive transistor. As a result, the gate-to-source voltage Vgs of the drive transistor changes, changing the current flowing through the drive transistor. This changes the current flowing through the organic EL element, changing the emission brightness of the organic EL element.
With a pixel circuit using a polysilicon TFT, on the other hand, a threshold voltage Vth of the drive transistor and a mobility μ of the semiconductor thin film making up the channel of the drive transistor change with time in addition to the secular deterioration of the I-V characteristic of the organic EL element. Moreover, the threshold voltage Vth and the mobility μ may be different between different pixels due to a manufacturing process variation (that is, different transistors exhibit different characteristics).
In the event of a difference in the threshold voltage Vth of the drive transistor or the mobility μ, the current flowing through the drive transistor changes. This leads to a change in emission brightness of the organic EL element between different pixels even if the same voltage is applied to the gate of the drive transistor, thus impairing the uniformity over the screen.
For this reason, each of the pixel circuits has various compensation and correction functions to ensure that the emission brightness of the organic EL element remains constant even in the event of a secular deterioration of the I-V characteristic of the organic EL element or a secular change in the threshold voltage Vth or the mobility μ of the drive transistor without being affected by such a change or deterioration. One of the functions is the compensation function adapted to compensate for the change in characteristic of the organic EL element. Another function is the correction function adapted to correct the change in the threshold voltage Vth of the drive transistor (hereinafter written as “threshold correction”). Still another function is the correction function adapted to correct the mobility μ of the drive transistor (hereinafter written as “mobility correction”) (refer, for example, to Japanese Patent Laid-Open No. 2006-133542).